This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-350419, filed Nov. 15, 2001, No. 2002-060791, filed Mar. 6, 2002; and No. 2002-120761, filed Apr. 23, 2002, the entire contents of all of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an injection molding machine in which a linear motor is used as a driving source of a linear motion member and, particularly, to the construction of a mechanism to axially drive a screw in an injection unit of an electric injection molding machine.
2. Description of the Related Art
In an injection molding machine which performs molding by injecting molten resin into dies, linear drive mechanisms are used in many places such as an injection unit which axially moves a screw in a heating barrel, a die clamping unit which performs opening and closing of the dies and die clamping, and an ejector which pushes out a product from the dies. When linear motions are electrically performed, a mechanism which converts the rotational motion of a motor to a linear motion by means of a ball screw has hitherto been widely used.
In contrast, Jpn. Pat. Appln. KOKAI Publication No. 63-60720 discloses an injection molding machine in which an alternating-current linear motor is used as a driving source of a linear motion member. Because the linear motion member is directly driven by using the alternating-current linear motor, a mechanism to convert a rotational motion to a linear motion becomes unnecessary and hence the construction of a linear drive mechanism can be simplified.
However, the following problems arise when an alternating-current linear motor is used as a driving source of a linear motion member of an injection molding machine:
(a) In general, an alternating-current linear motor has a large total length in the axial direction in comparison with a ball screw and, therefore, this results in a large drive mechanism and a cost increase.
(b) A linear motor with an iron core coil is used to obtain a large thrust. However, a strong mechanical structure becomes necessary because of the generation of a magnetic attraction which is as much as several times the thrust and, therefore, this results in an increase in the size of the drive mechanism, a weight increase, an increase in complexity and a cost increase.
(c) Due to the large magnetic attraction, electrical phase angle positioning is difficult and it is impossible to reduce phase differences. Such phase differences cause disturbances and pose the problems that it is difficult to increase gains and that vibrations occur.
The present invention was made in view of the above-described problems in the driving source of a linear motion member of a conventional injection molding machine. The object of the invention is to provide an injection molding machine in which a drive mechanism that is compact and has a large thrust is used as a driving source of a linear motion member. Particularly, the object of the invention is to provide an injection molding machine in which such a drive mechanism is used as a driving source to axially drive a screw of an injection unit.
An injection molding machine of the invention comprises: a heating barrel having a nozzle at the leading end thereof and being connected to the rear surface of dies through the nozzle; a screw being incorporated in the heating barrel and injecting molten resin into the dies by advancing in the heating barrel; and one or more than two voice-coil linear motors being connected to the tail end of the screw and axially driving the screw in the heating barrel.
In an injection molding machine of the invention, a voice-coil linear motor, which is widely used as the source of vibration in a speaker, is used as a driving source of the screw of an injection unit. A voice-coil linear motor, which is a direct-current linear motor, is constituted by a coil having an axis parallel to the driving direction and a magnet arranged around (or inside) the coil. When a direct current flowing in the coil interacts with the magnetic field generated by the magnet, a force is generated between the magnet and the coil and the magnet (hence, a moving body connected to the magnet) moves along the axis of the coil. The thrust of a voice-coil linear motor is determined by the current flowing in the coil.
A voice-coil linear motor has the following features:
(a) Because of the high utilization rate of magnetic flux in comparison with an alternating-current linear motor, a larger thrust can be obtained with the same size. Therefore, the size and the total length are small. Besides, it is easy to increase rigidity and the construction is simple.
(b) Because the coil and the magnet are coaxial, an imbalance in magnetic attraction is less apt to occur. For this reason, a construction to resist magnetic attraction is not required and, therefore, a light, highly rigid construction is possible and manufacturing at low cost is possible.
(c) Because of the direct-current motor, there is no phase difference.
(d) When the magnet is provided on the moving side the coil is provided on the fixed side, in this case it is unnecessary to consider the flexibility of electric wiring.
Therefore, by using a voice-coil linear motor as a driving source of the screw of an injection unit in an injection molding machine, the construction of the injection unit can be simplified and the total length of the injection unit can be made short.
When a large thrust is required in an injection molding machine of the invention, a plurality of voice-coil linear motors are connected to the tail end of the screw in parallel and arranged around the axis of the screw to form axial symmetry.
By arranging the voice-coil linear motors like this, the generation of a couple of forces around the shaft of the screw can be prevented. This removes the main factors in excessive loads on the connections between the linear motors and the screw and on various parts of the linear motors and, therefore, the construction of these parts can be simplified and the weight of these parts can be reduced.
In this case, preferably, the voice-coil linear motors are configured in such a manner that each of the motors can be independently controlled.
By configuring the voice-coil linear motors like this, it is possible to appropriately select the number of linear motors according to the type of product to be manufactured. This permits fine control of thrust during injection and can save electric power consumption.
Incidentally, in the above-described injection molding machine, it is also possible to use, instead of each of the voice-coil linear motors, a linear motor unit which is constituted by a plurality of voice-coil linear motors connected in parallel. As with the foregoing case, these linear motor units are also connected to the tail end of the above-described screw in parallel and arranged around the axis of the screw to form axial symmetry.
Also in this case, preferably, the linear motor units are configured in such a manner that each of the linear motor units can be independently controlled.
Incidentally, it is also possible to use the voice-coil linear motor as a driving source of a plunger in an injection unit of a plunger preplasticating type, a driving source of an ejector which pushes out a product from dies, a driving source of a movable platen of a die clamping unit which performs the opening and closing of dies and die clamping, etc., in addition to a driving source of a screw in an injection unit of an in-line screw type as described above.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.